1. Field of the Invention
This invention relates to an improved rotatable ball valve designed for operation under high differential pressures, such as are encountered in a downhole valve for an oil well.
2. Description of the Prior Art
Rotatable ball valves are well known in the valve art. In particular, there are prior art disclosures of a trunnion-mounted ball valve which cooperates with a pressure-biased seal seat which seals on the upstream side of the ball. In this type of valve, the thrust of the ball is carried by the trunnion bearings, thereby reducing the load on the seal seat. However, when this type of ball valve is opened, a large differential pressure across the ball valve will subject the rotating mechanism to damage due to large frictional resistance to rotation exerted by the seal, and also exposes the ball seal to tearing, extrusion or fluid erosion. As the ball rotates from the closed to the open position and the edge of the valve port in the ball passes the seal surface of the valve seat (i.e., the valve has just "cracked" open), a small portion of the valve seat is exposed to flow velocity great enough to quickly damage the hardest of materials. A similar high velocity flow occurs on closing the valve.
To overcome these deleterious effects, a prior art construction has provided a trunnion-mounted ball having an elastomeric ball seat but providing a mechanism wherein each opening movement of the valve rotates the ball seat several degrees each time the ball is closed. This does not eliminate the valve seat damage caused by erosive high velocity fluid during opening or closing, but it does expose a new section of the seat to the high velocity flow each time the ball is opened and closed, and extends the seal life by distributing the wear around the entire seal. Such mechanism does not, of course, have any reducing effect on the torque required to rotate the ball.
Another example of the prior art is a conventional subsurface safety valve. This valve has an elastomeric primary seal and an adjacent secondary metal seal, but it is a floating ball type valve, and hence, the thrust of the ball is carried by the ball seats. The feature of this valve is that the metallic ball seat is actuated to move the ball away from the elastomeric primary seal before rotation of the ball is initiated. This necessarily means that the ball is rotated while in contact with the metal seal. This construction thus requires some apparatus for achieving an equalization of the pressure differential across the ball valve before attempting to rotate the ball against the metallic ball seat, or damage to the rotating mechanism from excessive torque and to the ball seat by fluid erosion will occur.